1. Field of the Invention
Embodiments of the invention relate to a compact Mid-Infrared (MIR) laser which finds applications in many fields such as, molecular detection and imaging instruments for use in medical diagnostics, pollution monitoring, leak detection, analytical instruments, homeland security and industrial process control. Embodiments of the invention are also directed more specifically to the detection of molecules found in human breath, since such molecules correlate to existing health problems such as asthma, kidney disorders and renal failure.
2. Description of Related Art
MIR lasers of interest herein may be defined as, lasers having a laser output wavelength in the range of approximately 3-12 μm (3333-833 cm−1). More broadly, however, “MIR” may be defined as wavelengths within a range of 3-30 μm. The far-IR is generally considered 30 300 μm, whereas the near IR is generally considered 0.8 to 3.0 μm. Such lasers are particularly advantageous for use in absorption spectroscopy applications since many gases of interest have their fundamental vibrational modes in the mid-infrared and thus present strong, unique absorption signatures within the MIR range.
Various proposed applications of MIR lasers have been demonstrated in laboratories on bench top apparatuses. Actual application of MIR lasers has been more limited and hampered by bulky size and cost of these devices.
One laser gain medium particularly useful for MIR lasers is the quantum cascade laser (QCL). Such lasers are commercially available and are advantageous in that they have a relatively high output intensity and may be fabricated to provide wavelength outputs throughout the MIR spectrum. QCL have been shown to operate between 3.44 and 84 μm and commercial QCL are available having wavelengths in the range of 5 to 11 μm. The QCL utilized two different semiconductor materials such as InGaAs and AlInAs (grown on an InP or GaSb substrate for example) to form a series of potential wells and barriers for electron transitions. The thickness of these wells/barriers determines the wavelength characteristic of the laser. Fabricating QCL devices of different thickness enables production of MIR laser having different output frequencies. Fine tuning of the QCL wavelength may be achieved by controlling the temperature of the active layer, such as by changing the DC bias current. Such temperature tuning is relatively narrow and may be used to vary the wavelength by approximately 0.27 nm/Kelvin which is typically less than 1% of the of peak emission wavelength.
The QCL, sometimes referred to as Type I Cascade Laser or Quantum Cascade Laser, may be defined as a unipolar semiconductor laser based on intersubband transitions in quantum wells. The QCL, invented in 1994, introduced the concept of “recycling” each electron to produce more than one photon per electron. This reduction in drive current and reduction in ohmic heating is accomplished by stacking up multiple “diode” regions in the growth direction. In the case of the QCL, the “diode” has been replaced by a conduction band quantum well. Electrons are injected into the upper quantum well state and collected from the lower state using a superlattice structure. The upper and lower states are both within the conduction band. Replacing the diode with a single-carrier quantum well system means that the generated photon energy is no longer tied to the material bandgap. This removes the requirement for exotic new materials for each wavelength, and also removes Auger recombination as a problem issue in the active region. The superlattice and quantum well can be designed to provide lasing at almost any photon energy that is sufficiently below the conduction band quantum well barrier.
Another type of Cascade Laser is the Interband Cascade Laser (ICL) invented in 1997. The ICL, sometimes referred to as a Type II QCL (Cascade Laser), uses a conduction-band to valence-band transition as in the traditional diode laser, but takes full advantage of the QCL “recycling” concept. Shorter wavelengths are achievable with the ICL than with QCL since the transition energy is not limited to the depth of a single-band quantum well. Thus, the conduction band to valance band transitions of the Type II QCLs provide higher energy transitions than the intra-conduction band transitions of the Type I QCLs. Typical wavelengths available with the Type II QCL are in the range of 3-4.5 μm, while the wavelengths for the Type I QCLs generally fall within the range of 5-20 μm. While Type II QCLs have demonstrated room temperature CW operation between 3.3 and 4.2. μm, they are still limited by Auger recombination. Clever bandgap engineering has substantially reduced the recombination rates by removing the combinations of initial and final states required for an Auger transition, but dramatic increases are still seen with active region temperature. It is expected that over time improvements will be made to the ICL in order to achieve the desired operating temperature range and level of reliability.
For purposes of the present invention, QCL and ICL may be referred to under the generic terminology of a “quantum cascade laser” or “quantum cascade laser device”. The laser gain medium referred to herein thus refers to a quantum cascade laser. In the event that it is needed to distinguish between QCL and ICL, these capitalized acronyms will be utilized.
For the purposes of the present invention, the term “subband” refers to a plurality of quantum-confined states in nano-structures which are characterized by the same main quantum number. In a conventional quantum-well, the subband is formed by each sort of confined carriers by variation of the momentum for motion in an unconfined direction with no change of the quantum number describing the motion in the confined direction. Certainly, all states within the subband belong to one energy band of the solid: conduction band or valence band.
For the purposes of the present invention, the term “nano-structure” refers to semiconductor (solid-state) electronic structures including objects with characteristic size of the nanometer (10−9) scale. This scale is convenient to deal with quantum wells, wires and dots containing many real atoms or atomic planes inside, but being still in the size range that should be treated in terms of the quantum mechanics.
For the purposes of the present invention term “unipolar device” refers to devices having layers of the same conductivity type, and, therefore, devices in which no p-n junctions are a necessary component.
The development of small MIR laser devices has been hampered by the need to cryogenically cool the MIR lasers (utilizing, for example, a large liquid nitrogen supply) and by the relatively large size of such devices hampering their portability and facility of use and thus limiting their applicability.